This application claims the priority of Japanese Patent Application No. 8-320908 filed on Nov. 14, 1996, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a signal processor circuit for electronic endoscope systems which read out all picture elements of images accumulated by image pickup devices and form still images of high qualities.
2. Prior Art
An electronic endoscope system uses a CCD (charge coupled device), for example, as an image pickup device which obtains video signals by reading out electric charges accumulated per pixel with photoelectric converter elements. In a simultaneous type electronic endoscope system, color filters are arranged per pixel on a top surface of the CCD for obtaining a colored image.
An arrangement condition of the color filters is illustrated in FIG. 6(A), wherein Mg (magenta), G (green), Cy (cyanic) and Ye (yellow), for example, are arranged per pixel on an top surface of a CCD 1. Accordingly, electric charges are accumulated in the CCD 1 from rays which have transmitted through these color filters.
FIG. 6(B) shows a readout condition in the conventional 2-line mixed reading method, in which stored electric charges of pixels in an upper line and a lower line are read out in a mixed condition.
For example, video signals in Odd fields such as a mixture signal of lines Nos. 0 and 1, a mixture signal of lines Nos. 2 and 3, . . . are read out at a first exposure time, whereas video signals in Even fields such as a mixture signal from lines Nos. 1 and 2, a mixture signal from lines Nos. 3 and 4, . . . are read out at a second exposure time. Accordingly, the mixture signal from the two lines on a CCD 1 is used as a signal from a single line on a field image.
FIG. 7 shows signal readout operations in the CCD 1, in which Odd fields and Even fields are obtained at intervals of 1/60 second (a vertical synchronizing period) as indicated by a field O/E signal in (A) of FIG. 7.
Accordingly, signals are accumulated for an accumulating (exposure) time t of an electronic shutter within the period of 1/60 second described above, for example, as shown in (B) of FIG. 7 and the accumulated mixture signals are read out during the next period of 1/60 second. As a result, an Odd field signal (No.) n-1 and an Even field signal n are obtained as shown in (C) of FIG. 7, whereby the Odd field signal n-1 is composed of mixture signals from lines Nos. (0+1), (2+3), (4+5), . . . which are shown on the left side in (B) of FIG. 6, whereas the Even field signal n is composed of mixture signals from lines Nos. (1+2), (3+4), . . . which are shown on the right side in (B) of FIG. 6.
The Odd field signal and the Even field signals are subjected to an interlace scanning and formed into an image within a single frame, which is displayed on a monitor.
However, the simultaneous type electronic endoscope system allows a time deviation of 1/60 second to be produced between an Odd field image and an Even field image which are to be used for forming an image within a single frame as shown in (C) of FIG. 7, thereby posing a problem that an image quality is lowered when the endoscope itself vibrates or an object to be observed moves during this time deviation. Endoscopes in particular are equipped with freeze switches and display still images through manipulation of these switches for permitting detailed observation of specific locations. Since a frame of such a still image is displayed on the basis of the Odd and Even field signals described above, a degradation of image quality (including discoloration) is caused when a deviation (vibration or movement) is caused between the Odd and Even field images.
In processing of the video signals obtained from an image pickup device, a clamp which matches black levels is performed as known well for cancelling a difference between amplification degrees of the signals, etc. and the clamp processing has an important role for maintaining a favorable image quality.